Energy management foams are generally considered foams which compress upon impact so as to absorb considerable impact energy over a relatively long time period. Such foams have been increasingly important in the passenger compartment of transportation vehicles, and may be found in arm rests, head rests, pillars, roof rails, dashboards, knee bolsters, side bolsters, bumpers, and the like. Many of these parts are visible to the occupant, being covered with polyvinyl, fabric, or leather trim materials.
The energy managed, EM, is the integral of the force (F) versus deflection curve up to a compression limit of 40%: ##EQU1## where d=deflection at 40% compression.
Energy management foams must also be efficient. Energy management efficiency, E, reflects the difference between an ideal square wave energy, the product of force and deflection, and the energy managed. E, in percent, is defined as EQU E=[EM(d)/F(d).multidot.d].multidot.100,
or ##EQU2##
Flexible foams are not efficient energy absorbers, and virtually all energy management applications in vehicle interiors employ rigid polyurethane foam. Typical of formulations used to produce such foams are U.S. Pat. Nos. 5,143,941; 5,187,204; 5,248,703; 5,248,704; and 5,453,455, and the patents cited therein. For example, U.S. Pat. No. 5,143,941 discloses rigid polyurethane foams which are water blown and exhibit a relatively constant strength to crush. While such foams are excellent energy absorbing foams, their energy absorption is due to irreversibly deforming and breaking the foam cell walls during compression. Thus, following impact, such foams are unable to recover their initial shape. More importantly, the crushed foam is unable to again provide energy management. This defect is exceedingly important should a severe impact later occur. It must be stressed that the loss of energy management ability may occur from only incidental or mild repeated contact.
In the foregoing patents, typical formulations require special polyols derived from the oxyalkylation of toluene diamine or ethylene diamine, or, in the case of U.S. Pat. No. 5,453,455, the use of lithium salts or formic acid, (both environmentally suspect). Polymer polyols are somewhat uniformly disclosed as unsuitable in these references in any concentrations. See, for example, U.S. Pat. No. 5,143,941 in this regard.
U.S. Pat. No. 5,216,041 is directed to similar energy absorbing foam compositions wherein minor amounts of polymer polyols are included along with amine-based polyols. However, the '041 patent indicates that when more than 30% by weight of polymer polyols are used, the foams fail as energy absorbing foams. For the polymer polyols employed (40% solids), 30% polymer polyol corresponds to an upper total solids limit of less than 10 weight percent in the overall formulation.
U.S. Pat. Nos. 4,116,893 and 4,212,954 disclose energy management foams with a decreased temperature dependence on energy absorption efficiency. Both patents employ polymer polyols together with relatively large amounts (in equivalents) of low molecular weight crosslinkers such as ethylene glycol, diethylene glycol, 1,4-butanediol, trimethylolpropane, and the like. The '893 patent adopts a prepolymer approach which is not optimal due to the extra expense in preparing the isocyanate-terminated prepolymers. The use of relatively large molar equivalents of low molecular weight chain extenders increases the isocyanate requirement, which further increases costs. Total polymer solids in both patents are very limited, following the teachings of U.S. Pat. Nos. 5,143,941; and 5,216,041. In the '893 patent, typical formulations employ about 8 weight percent polymer solids, while in the '954 patent, about 10.8 weight percent polymer solids are employed.
The result of the irreversible crushing of the rigid energy management foams of the prior art from an aesthetic point of view is that the surface of the door panel, dashboard, etc., will appear permanently dented or deformed. Many such parts, e.g., dashboards, are very expensive to replace. In the case of side bolsters, which are encased within the vehicle door, impact damage may not be visible to the occupant. There is a high probability, however, that upon replacement of door panels damaged in an accident, that the side bolsters, which are generally poured-in-place at the factory, will not be replaced. Thus, the repaired vehicle will no longer meet safety requirements, unknown to the owner. Again, it must be stressed that foams without high recovery can be crushed even by relatively minor impact, compromising the ability of the foam to absorb energy in severe impacts.